Our research program has developed a novel, trimodal gene therapy-based approach for the treatment of cancer. Our approach utilizes a cytolytic, replication-competent adenovirus (Ad5-CD/TKrep) to selectively and efficiently deliver a pair of therapeutic "suicide" genes to tumors. Our preclinical studies have demonstrated that the Ad5-CD/TKrep virus itself, via its cytolytic activity, has potent anti-tumor activity. The efficacy of Ad5-CD/TKrep viral therapy can be enhanced significantly by invoking two suicide gene systems (CD/5-FC and HSV-1 TK/GCV), which render malignant cells sensitive to specific pharmacological agents and importantly, sensitizes them to radiation. A major objective of Project 1 is to develop second-generation adenoviruses that may be more efficacious and less toxic than the parental Ad5-CD/TKrep virus. We will determine whether second-generation adenoviruses containing various E3 genes demonstrate greater anti-tumor activity in an immune-competent host relative to the parental Ad5-CD/TKrep virus. We will test the hypothesis that suppression of the host immune response by E3 genes will result in longer-term therapeutic gene expression and improved tumor control. Anti-tumor activity will be correlated with the duration of therapeutic gene expression in vivo and the extent, and nature, of the immune response. We will determine whether second-generation adenoviruses expressing a more catalytically active yeast CD/mutant HSV-1 TK(SR39) transgene results in better tumor control than the parental Ad5-CD/TKrep virus. We will examine the toxicity of second-generation adenoviruses in the immune-competent mouse following intraprostatic and intravenous administration. Finally, we will develop a series of second-generation adenoviruses expressing the human sodium iodide symporter (hNIS). We will test the hypothesis that expression of hNIS will enable virus-infected cells to take up (99m)TcO4- and (123)I and allowing for non-invasive monitoring of vector biodistribution and therapeutic gene expression in vivo. All of the second-generation adenoviruses developed in Project 1 will be used in the other projects of this Program.